This invention generally relates to methods and apparatus for forming fins on the outside surface of a tube, and more specifically, to methods and apparatus particularly well suited for forming high pitch fins on the outside surface of a tube.
In many heat transfer applications, a first fluid is conducted through the inside of a tube and a second fluid is conducted over the exterior thereof to transfer heat between the first and second fluids. Many factors, for example the thickness of the tube wall and the material from which the tube is made, affect the heat transfer performance of the tube. The amount of outside surface area on a tube also affects the heat transfer characteristics thereof; and as a general rule, increasing this surface area on a tube improves the heat transfer performance thereof.
The outside surface area of a tube can be, and often is, increased by forming external fins on the outside of the tube. Generally, increasing the number of fins per unit length of a tube--a ratio referred to herein as the fin pitch of the tube--increases the outside surface area of the tube and, hence, improves the heat transfer characteristics thereof. However, increasing the fin pitch of a tube also usually increases the cost of forming the tube fins; and with prior art tube finning methods and apparatus, it has generally not been cost effective to provide tubes with a fin pitch greater than approximately 16 fins per centimeter (40 fins per inch). That is, with prior art tube finning methods and apparatus, the increased cost of providing a tube with a fin pitch greater than approximately 16 fins per centimeter has typically not been justified by the improved heat transfer performance of the tube.
To elaborate, conventional tube finning apparatus comprise a triad of substantially identical roller assemblies. Each roller assembly, in turn, comprises a rotatable arbor and a plurality of contiguous discs mounted on the arbor for unitary rotation therewith. These discs have uniform widths and progressively increasing outside diameters in a forward direction. The roller assemblies are positioned in a finning machine with the longitudinal axes of the roller assemblies laterally spaced from, substantially equally spaced around, and slightly skewed relative to a central longitudinal axis along which a tube is advanced during a finning operation. To form fins on the outside surface of a tube, the tube is aligned with this central longitudinal axis, between the roller assemblies; and then the roller assemblies are moved toward the tube, bringing the rearwardmost or lead discs of the roller assemblies into pressure engagement with the outside surface of the tube. Each roller assembly is then rotated about its own axis, rotating the lead discs of the roller assemblies against the tube surface.
This rotation of the lead discs rotates the tube about its own longitudinal axis and forms one or more small helical groove or grooves in the outside surface of the tube, with the number of grooves that are formed determined by the angle between the longitudinal axes of the tube and the roller assemblies. The rotation of the lead discs against the tube, because of the skewed angle between the longitudinal axes of the tube and the roller assemblies, also longitudinally advances the tube so that the formed groove or grooves helically extend around the outside surface of the tube. As the tube longitudinally moves forward, each groove is sequentially engaged by successive discs having successively increasing outside diameters. These discs enlarge the groove or grooves and extrude tube material outward, between adjacent discs. The outside annular surface of each disc has a curved or arcuate profile, curving radially inward from a central annular edge, and the arcuate profiles of adjacent discs shape the extruded tube material into a helical fin or fins extending around the outside surface of the tube.
With this arrangement, the distance between corresponding points of adjacent groove convolutions and between corresponding points of adjacent fin convolutions along a longitudinal cross section of the tube equals the width of the individual finning discs. Since the distance between corresponding points of adjacent fin convolutions is inversely proportional to the fin pitch, the fin pitch of a tube can be increased by using thinner discs to form the tube grooves and fins.
However, the discs employed in the tube finning operation are subjected to considerable stresses as they extrude material to form the tube fins and grooves; and decreasing the thickness of the individual discs, decreases the strength of those discs and tends to increase the number and frequency of broken discs. Should a disc break, of course, the finning process must be eventually terminated and the broken disc replaced. This is quite costly, not only because of the cost of the individual discs but also because of the time that the finning machine is rendered inoperable and because of the time of the machine operator which must be spent to replace the disc. As a practical matter, these costs are a very real factor limiting the widths of the discs that may be employed with prior art tube finning methods and apparatus and, consequently, limiting the pitch of the tube fins that can be formed by these methods and apparatus.
Moreover, with the above-described finning operation, for every revolution about its axis, the tube longitudinally moves forward a distance equal to an integral number of disc widths, usually one, two, or three disc widths. Hence, the forward speed of the tube through the finning machine is dependent on the widths of the discs; and decreasing the widths of the discs in order to increase the fin pitch, decreases the forward speed of the tube through the finning machine. This increases the length of time required to fin a given length of tube, reducing the efficiency of the tube finning machine and of the machine operator.